The invention is related to the field of nitride devices and in particular the fabrication of high performance normally-off nitride transistors.
Group III-Nitride semiconductors are very promising for power electronics applications due to their combination of high critical electric field (˜3.3 MV/cm) and high electron density (ne>1013 cm−2) and mobility (1500-2000 cm2/Vs) of the two-dimensional electron gas (2DEG) in the Al(In)GaN/GaN high mobility electron transistors (HEMTs). Comparing with Si power switches, more than three orders of magnitude reduction of specific on-resistance (Rsp,on) are expected from the GaN-based power switches. GaN-based power converters will enable more efficient and compact power conversion systems than the Si-based converters.
The standard Al(In)GaN/GaN HEMTs are depletion-mode (D-mode) devices. However, enhancement-mode (E-mode) transistors are highly desirable for power electronics as they can greatly simplify circuit designs and improve system reliability. The challenge of making E-mode GaN-based transistors is to maintain low on-resistance (Ron) while have a high breakdown voltage (By). Due to the reduced channel mobility and/or limited gate-voltage swing, the channel resistance is a major contribution to the on-resistance of the E-mode GaN-based transistors fabricated by methods in the prior arts, such as the gate recess. Reducing the gate length can reduce the channel resistance. However, the breakdown voltage is also reduced with short gate length.
A composite control circuit which includes a low voltage normally-off gating device connected in series with a normally-on high voltage device can be applied to nitride semiconductors to fabricate high voltage composite normally-off device. Since the normally-off device is a low voltage device, its on-resistance will only be a small portion of the composite device, while the high voltage normally-on nitride device can support the off-state high voltage. However, these two devices need to be brought together in a very close proximity to reduce parasitic.